Alternative σ Factors Regulate Overlapping as Well as Distinct Stress Response and Metabolic Functions in Listeria monocytogenes under Stationary Phase Stress Condition

Listeria monocytogenes can regulate and fine-tune gene expression, to adapt to diverse stress conditions encountered during foodborne transmission. To further understand the contributions of alternative sigma (sigma) factors to the regulation of L. monocytogenes gene expression, RNA-Seq was performed on L. monocytogenes strain 10403S and five isogenic mutants (four strains bearing in-frame null mutations in three out of four alternative sigma factor genes, Delta CHL, Delta BHL, Delta BCL, and Delta BCH, and one strain bearing null mutations in all four genes, Delta BCHL), grown to stationary phase. Our data showed that 184, 35, 34, and 20 genes were positively regulated by sigma(B), sigma(L), sigma(H), and sigma(c) (posterior probability > 0.9 and Fold Change (FC) > 5.0), respectively. Moreover, sigma(B)-dependent genes showed the highest FC (based on comparisons between the Delta CHL and the Delta BCHL strain), with 44 genes showing an FC > 100; only four sigma(L)-dependent, and no sigma(H)- or sigma(c)-dependent genes showed FC >100. While sigma(B)-regulated genes identified in this study are involved in stress-associated functions and metabolic pathways, sigma(L) appears to largely regulate genes involved in a few specific metabolic pathways, including positive regulation of operons encoding phosphoenolpyruvate (PEP)-dependent phosphotransferase systems (PTSs). Overall, our data show that (i) sigma(B) and sigma(L) directly and indirectly regulate genes involved in several energy metabolism-related functions; (ii) alternative sigma factors are involved in complex regulatory networks and appear to have epistatic effects in stationary phase cells; and (iii) sigma(B) regulates multiple stress response pathways, while sigma(L) and sigma(H) positively regulate a smaller number of specific pathways.

Automated summary

Listeria monocytogenes can regulate gene expression to adapt to different stress conditions during foodborne transmission, with alternative sigma factors playing a key role. Sigma(B) and sigma(L) were found to have significant effects on regulating genes related to energy metabolism, while sigma(H) and sigma(c) had more limited effects on specific pathways.